In the art of membrane-type tanks, the internal surfaces of a bearing structure such as the internal hull of a double-hulled ship are lined with a multilayered structure comprising two thin sealing membranes alternating with two layers of thermal insulation which serve both to limit heat flux through the tank wall and to provide structure support for the sealing membranes.
In order to maximize operational yield of such a tank it is desirable to optimize the usable volume of cargo that can be loaded into the tank and unloaded from the tank. However, the use of an unloading pump that draws the liquid toward the top of the tank means that a certain head of liquid needs to be left in the bottom of the tank otherwise the suction member of the pump will come into communication with the gaseous phase which will cause the pump to become airbound and/or damaged. Given the way in which the cargo is sloshed around by the swell, the head of liquid required can be minimized only with difficulty.
Publication FR-A-2832783 envisions the creation of a sump in the cryogenic insulation of the tank as being a solution that is expensive and rather ineffectual.
Publication KR-10-2010-0092748 discloses a sump obtained by creating a concave stepped portion in the bottom wall of a membrane-type tank. This stepped portion nevertheless still has difficulties in realizing it given the need to divert the entire multilayer structure of the tank wall into the concave stepped portion.
FR1318891 describes a self-supporting metal tank for liquefied gas which is positioned in a ship with the interposition of a thick layer of thermal insulation between the self-supporting metal tank and the interior hull of the ship. In one embodiment, a lateral wall of the metal tank is pierced with an exhaust orifice opening into a pipe connecting the metal tank to a pump sump by means of a pickup pipe and control valves or cocks. In another embodiment, a bottom wall of the metal tank is pierced with an orifice opening into a pipe connecting the metal tank to a pump sump via a pickup pipe and control valves or cocks. A centrifugal pump arranged in the pump sump allows the liquid to be displaced by causing it to pass through a riser intended to be connected to an on-shore facility. The pump sump, the centrifugal pump and the riser are arranged outside the metal tank, notably between two walls of a transverse bulkhead of the ship where they are readily accessible.